Understanding the process-microstructure correlations for tailoring the mechanical properties of L-PBF produced austenitic advanced high strength steel

In this work, the additive manufacturing technique of laser powder bed fusion (L-PBF) was used to build up X30Mn21 austenitic advanced high strength steel (AHSS) samples. Different L-PBF process parameters were used to understand the correlation between process, microstructure, texture, and mechanical properties. The influence of build platform preheating (200 °C-800 °C), laser speed (550 mm/s - 950 mm/s) and scan strategy (bidirectional continuous and Mark&Sleep (M&S)) on grain size, grain morphology, size of solidification cells, dislocation density, and texture was studied. Local solidification parameters in the melt pool e.g. cooling rates, temperature gradients and solidification velocities were simulated by a FEM heat flow model and correlated with the solidification microstructure. By using SEM/EBSD analysis and tensile testing, the mechanical properties of the AHSS were assessed by considering microstructural aspects. It was found that AHSS, produced with higher laser speeds and an alternative M&S scan strategy, revealed a reduced grain size and texture intensity. This was attributed to a partial columnar to equiaxed transition (CET), as well as a significantly increased density of geometrically necessary dislocations. Preheating of the build platform promoted columnar grain growth with a more pronounced texture, low dislocation densities, and reduced yield strength. The influence of cooling rate, temperature gradient and solidification velocity on microstructural and textural evolution is discussed based on fundamental solidification theories.